The present invention relates to a nonvolatile memory cell having a capacitor formed by the use of a ferroelectric film, and also to a method of manufacturing the same.
With recent progress of the film forming technology, rapid studies are currently advanced in the application of nonvolatile memory cells each using a ferroelectric thin film. In the nonvolatile memory cell, it is possible to perform fast polarization inversion of a ferroelectric thin film and also fast rewriting effected by utilizing the residual polarization of the thin film. The nonvolatile memory cells with ferroelectric thin films being currently studied are broadly classified into a type to detect a change of a stored charge amount in a dielectric capacitor, and a type to detect a resistance change caused by spontaneous polarization of a ferroelectric. The semiconductor memory element related to the present invention belongs to the former type.
It is an object of the present invention to provide a semiconductor memory element of an improved construction so contrived as to suppress diffusion of hydrogen or intrusion of water content into a capacitor having a ferroelectric thin film, and also to provide a method of manufacturing such an improved memory element.
For the purpose of achieving the object mentioned above, the capacitor structure of the semiconductor memory element according to the present invention comprises: (a) a lower electrode formed on a semiconductor substrate; (b) a capacitor part composed of a ferroelectric thin film formed on the lower electrode; and (c) an upper electrode formed on the capacitor part. The capacitor structure further has: (d) a first protective layer consisting of one or more layers formed between the semiconductor substrate and the lower electrode, and composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, silicon nitride, nickel and palladium; and (e) a second protective layer consisting of one or more layers formed on the upper electrode, and composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, nickel and palladium.
In the capacitor structure of the semiconductor memory element of the present invention, it is preferred that the upper electrode covers the capacitor part, the lower electrode and the first protective layer via an insulator layer. In this case, it is further preferred that the second protective layer covers the surface of the upper electrode. Here, the expression of covering the surface of the upper electrode with the second protective layer does not signify that, when the upper electrode functions also as a wiring (e.g. plate line), the second protective layer covers even such wiring as well. This definition applies to the following description also.
A method of forming a capacitor structure of a semiconductor memory element according to a first aspect of the present invention for achieving the above object comprises the steps of: (a) forming, on a semiconductor substrate, a first protective layer consisting of one or more layers composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, silicon nitride, nickel and palladium; (b) forming a lower electrode layer on the first protective layer; (c) forming a lower electrode by patterning the lower electrode layer and the first protective layer; (d) forming a ferroelectric thin film on the lower electrode, and patterning the ferroelectric thin film to thereby form a capacitor part composed of the ferroelectric thin film; (e) forming an insulator layer on the entire surface, and then forming an opening in the insulator layer at a position above the capacitor part; (f) forming an upper electrode layer on the insulator layer inclusive of the opening; (g) forming a second protective layer consisting of one or more layers formed on the upper electrode layer, and composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, nickel and palladium; and (h) forming an upper electrode by patterning the second protective layer and the upper electrode layer.
A method of forming a capacitor structure of a semiconductor memory element according to a second aspect of the present invention for achieving the above object comprises the following steps instead of the steps (f), (g) and (h) defined in the method according to the second embodiment: (i) forming an upper electrode layer on the insulator layer inclusive of the opening, and then patterning the upper electrode layer to thereby form an upper electrode; and (j) forming, on the upper electrode, a second protective layer consisting of one or more layers and composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, nickel and palladium, and then patterning the second protective layer in such a manner that the surface of the upper electrode is covered with the second protective layer.
A method of forming a capacitor structure of a semiconductor memory element according to a third aspect of the present invention for achieving the above object comprises the following steps instead of the steps (c) and (d) defined in the method according to the first embodiment: (k) forming a ferroelectric thin film on the lower electrode layer; and (l) patterning the ferroelectric thin film, the lower electrode layer and the first protective layer to thereby form a capacitor part composed of the ferroelectric thin film, and a lower electrode.
A method of forming a capacitor structure of a semiconductor memory element according to a fourth aspect of the present invention for achieving the above object comprises the steps of: (a) forming, on a semiconductor substrate, a first protective layer consisting of one or more layers composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, silicon nitride, nickel and palladium; (b) forming a lower electrode layer on the first protective layer; (c) forming a ferroelectric thin film on the lower electrode layer; (d) patterning the ferroelectric thin film, the lower electrode layer and the first protective layer to thereby form a capacitor composed of said ferroelectric thin film, and a lower electrode; (e) forming an insulator layer on the entire surface, and then forming an opening in the insulator layer at a position above the capacitor part; (f) forming an upper electrode layer on the insulator layer inclusive of the opening, and then patterning the upper electrode layer to thereby form an upper electrode; and (g) forming, on the upper electrode, a second protective layer consisting of one or more layers and composed of a material selected from those of Group IVa transition metal, Group Va transition metal, Group IVa transition metal nitride, Group Va transition metal nitride, nickel and palladium, and then patterning the second protective layer in such a manner that the surface of the upper electrode is covered with the second protective layer.
In the present invention, an exemplary material for composing the lower electrode may be Pt, RuO2, IrO2, Laxe2x80x94Srxe2x80x94Coxe2x80x94O (LSCO) having a perovskite structure, or a two-layer structure of LSCO/Pt formed in this order from below. Meanwhile, an exemplary material for composing the upper electrode may be Pt, RuO2, IrO2 or aluminum alloy.
The ferroelectric thin film may be composed of a perovskite type ferroelectric material of a Bi-series layer structure. More specifically, an exemplary material suited for composing the ferroelectric thin film may be Bi2SrTa2O9, Bi2SrNb2O9, Bi2BaTa2O9, Bi4SrTi4O15, Bi4Ti3O12, Bi2SrTaxNb2xe2x88x92xO9, or Bi2PbTa2O9. Particularly it is preferred that the ferroelectric thin film be composed of a Y1-series material (Bi2(Sr, Ba, Ca) (Ta, Nb)2O9). And it is further preferred that the Y1-series material be composed of Bi2SrTa2O9. Other exemplary materials suited for composing the ferroelectric thin film are PZT and PLZT.
The first protective layer or the second protective layer may consist of a single layer of a material selected from those of Group IVa transition metal (Ti, Zr, Hf), Group Va transition metal (V, Nb, Ta), Group IVa transition metal nitride, Group Va transition metal nitride, silicon nitride, nickel and palladium. The first or second protective layer may consist of a two-layer structure which is composed of the following materials deposited in the shown order from below.
Group IVa transition metal/Group Va transition metal
Group IVa transition metal/Group IV transition metal nitride
Group IVa transition metal/Group Va transition metal nitride
Group Va transition metal/Group IVa transition metal
Group Va transition metal/Group IVa transition metal nitride
Group Va transition metal/Group Va transition metal nitride
Group IVa transition metal nitride/Group IVa transition metal
Group IVa transition metal nitride/Group Va transition metal
Group IVa transition metal nitride/Group Va transition metal nitride
Group Va transition metal nitride/Group IVa transition metal
Group Va transition metal nitride/Group IVa transition metal nitride
Group Va transition metal nitride/Group Va transition metal
Further, the first or second protective layer may consist of a three-layer structure composed of the following materials for example.
Group IVa transition metal/Group IVa transition metal nitride/Group IVa transition metal
Group IVa transition metal/Group IVa transition metal nitride/Group Va transition metal
Group IVa transition metal/Group Va transition metal nitride/Group IVa transition metal
Group IVa transition metal/Group Va transition metal nitride/Group Va transition metal
Group Va transition metal/Group Va transition metal nitride/Group IVa transition metal
Group Va transition metal/Group Va transition metal nitride/Group Va transition metal
In the above examples of multilayer compositions, any transition metal or transition metal nitride may be replaced with silicon nitride, nickel or palladium.
The semiconductor substrate may be composed of any of known insulator materials such as SiO2, BPSG, PSG, BSG, AsSG, PbSG, SbSG, SOG, SiON, SiN, NSG and LTO; or an intermediate insulator layer composed by laminating such insulator materials. The substrate may also be an element isolating region having a LOCOS structure or a trench structure
Similarly, the insulator layer formed under the upper electrode may be composed of any of known insulator materials such as SiO2, BPSG, PSG, BSG, AsSG, PbSG, SbSG, SOG, SiON, SiN, NSG and LTO; or an intermediate insulator layer composed by laminating such insulator materials.
The structure of the semiconductor memory element according to the present invention can be embodied in an example where one of the source-drain regions and the upper electrode of the selective transistor constituting the semiconductor memory element are connected electrically to each other via a contact plug and a wiring, while the lower electrode is connected to a plate line; or in another example where one of the source-drain regions and the lower electrode of the selective transistor are connected electrically to each other via a contact plug, while the upper electrode is connected to a plate line. The former example is generally termed a planar semiconductor memory element, and the latter example is generally termed a stacked semiconductor memory element. In the stacked semiconductor memory element where one of the source-drain regions and the lower electrode of the selective transistor are mutually connected via the first protective layer, it is desired that the electric conductivity of the first protective layer be less than 0.01 xcexa9xc2x7cm or so. In this case, therefore, using silicon nitride for the first protective layer is not considered to be adequate.
In the present invention, the capacitor structure of the semiconductor memory element is surrounded with the first and second protective layers. And Group IVa transition metal or Group Va transition metal has properties to occlude hydrogen. Meanwhile Group IVa transition metal nitride, Group Va transition metal nitride, silicon nitride, nickel or palladium has properties to effectively prevent diffusion of hydrogen. In this case, the first and second protective layers composed of such materials have a barrier effect against intrusion of water content. Therefore, the provision of the first and second protective layers realizes effective suppression of diffusion and intrusion of hydrogen or water content into the capacitor structure at the time of executing a hydrogen or heat treatment. As a result, there occurs no deterioration of the polarization characteristic of the capacitor structure, hence enhancing the long-term reliability of the semiconductor memory element. Moreover, when the substrate is composed principally of SiO2, Group IVa transition metal or Group Va transition metal exerts a function of enhancing the adhesion between the lower electrode and the substrate.